Procedure and system for detecting a person&#39;s fall

ABSTRACT

A person under supervision wears a sensor consisting of at least one accelerometer ( 31 ) and a magnetometer ( 41 ), oriented in his vertical direction. A fall event is picked up when a significant and rapid oscillation of the acceleration signal coincides with a shift in the ambient magnetic field between two levels (between t=4000 and t=5000). Additional criteria that may also make use of the magnetometer enable the diagnosis to be made, and it is easier and safer to establish this than with accelerometers alone.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a procedure and system for detecting when aperson falls. It will be especially useful in medical institutionstaking care of elderly people.

2. Discussion of the Background

There already exists a wide range of systems and procedures that monitora person's activity and even detect a critical state requiringintervention. In general, one or more sensors are placed on the personand these continually transmit signals about his/her physical activity.Such signals typically take measurements of acceleration in one or moredirections: hence a fall appears as a sharp change, generally involvinga series of oscillations of short duration in at least one of theacceleration signals. An example of this is patent U.S. Pat.No.-B2-6,703,939.

One pitfall frequently encountered with systems of this kind is that itis difficult to determine what kinds of things have happened to theperson wearing the sensor from the signals that it transmits. If a fallis characterized by an acceleration in the vertical direction, thenactions involving bending or sitting give a similar response. When lyingdown the wearer naturally reproduces a movement that resembles a fall.Certain criteria distinguishing between an accidental fall and normalactivities have been identified, but they are inadequate. Henceimprovements are eagerly awaited by specialists. The inventionrepresents such an improvement. In the procedure detecting a fall,measurement of the acceleration signal is supplemented by measuringsignals of a different kind, whose combination gives a more reliableindication of the fall.

SUMMARY OF THE INVENTION

The invention involves using at least one magnetometer in the sensorworn by the person, in addition to the accelerometer. The magnetometermeasures the ambient magnetic field and hence gives a reading which iscompletely different from that of the accelerometer recording theactivity of the wearer.

The measurements yielded by the magnetometer are not sensitive toaccelerations experienced by the sensor but only to changes inorientation. The measurements yielded by the magnetometer depend on thesensor's orientation in space: for example, when there is no magneticdisturbance, such as the presence of a ferromagnetic object in theperson's vicinity, the projection of the Earth's magnetic field on eachof the axes recorded by the sensor is measured. In particular, it hasbeen noted that, during the instability period of the accelerationsignal which characterizes a fall and some normal activities of theperson, measurement of certain components of the magnetic-field signalshowed a gradual transition between two different levels of magneticfield measurement, stable before and after the instability period when afall actually occurred, and measurement of the acceleration signal andthe difference in the magnetic-field measurement both before and afterthe instability together offered a good chance of pinpointing the fallby eliminating a significant number of events normally confused with it.

The invention can be applied in many ways. In particular, furthercriteria can be added to supplement the appraisal fall events and sogive an even more reliable indication.

For signals measured and monitored continuously, these criteria maygenerally be useful if changes in the signals exceed a threshold for aparticular period of time, for example in terms of signal strength orenergy.

Signals that can be usefully measured are the vertical component ofacceleration and the vertical component of the ambient magnetic field,measured in a frame associated with the person—from head to foot—and notmeasured with respect to the Earth. Falling from an upright pose isalways accompanied by such a vertical acceleration, and measurement ofthe vertical component of the ambient magnetic field changes from astable value in the standing position to a value that is usuallydifferent after a fall, the vertical component associated with theperson then being part of the horizontal ground reading.

It is understood that, in order for the above and other criteria to bereliable, the instability period of the acceleration signal must befollowed by a stable signal of sufficient duration, which may imply aloss of consciousness; if not, the event associated with theinstability, whatever its nature, is not considered serious and isignored by the procedure.

Other criteria for detecting a fall may include detecting a horizontalposition when the acceleration signal is stable, for example bymeasuring the acceleration due to gravity as being effectively zero inthe person's vertical direction.

The criteria may also include recognizing certain normal events that arelikely to be confused with a fall. A common occurrence of this type iswhen the person goes to bed. In this instance and in others, therecognition criterion can include measuring a person's azimuth, that is,his/her orientation in the horizontal plane. In the circumstances inwhich the invention is usually used the wearer has only a small numberof places in which to lie down, and these can easily be locatedpreviously. A fall will produce any azimuth whatsoever unlike normalreclining where the person's orientation is characterized by a knownazimuth.

Discriminating normal events can be achieved not only by measuring thefinal state but also by examining an entire section of a signal, asevents such as going to bed are generally accompanied by regular actionswhich impart an electronic character, that is, an electronic signatureto the signals.

In general terms, a fall may be indicated by a numerical combinationweighted by the criteria deployed. If the numerical threshold isexceeded by this combination, a fall will be signalled and the alertgiven.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention involves another system capable of implementing theprocedure:

FIG. 1 illustrates use of the sensor,

FIG. 2 is an overview of the system,

FIG. 3, consisting of diagrams 3A, 3B and 3C, illustrates a basicinstance of detection,

FIG. 4, consisting of diagrams 4A, 4B and 4C, illustrates a specialinstance of detection in the procedure,

and FIG. 5 is a diagram summarizing the procedure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 represents a person 1 to be supervised according to the procedureand marked with a frame consisting of a vertical axis VT extended fromhead to foot, an antero-posterior axis AP projecting forward andmedio-lateral axis ML projecting sideways. The origin of the axes ispresumed to be on the body of person 1, for example on the chest or hipwhere there is a sensor 2, the arrangement of which is given in greaterdetail in FIG. 2. It contains three accelerometers 31, 32 and 33 whichmeasure the accelerations they undergo, including gravity, along thethree orthogonal axes of the frame, and three magnetometers 41, 42 and43 which measure the components of the ambient magnetic field,essentially the Earth's natural magnetic field, along the threeorthogonal axes of the frame. It is important to orient sensor 2correctly on person 1, so that the measurement axes of accelerometers31, 32 and 33 and of magnetometers 41, 42 and 43 coincide with thedirections in the frame of person 1. Alternatively, sensor 2 can beplaced in any orientation but should be measured using a calibrationmethod (by placing the person 1 in a fixed position with respect to theground and by using the measurements of the sensor 2 in this position),and the readings of the sensors would then be adjusted to align with thethree axes of the frame. The readings, adjusted or not, ofaccelerometers 31, 32 and 33 and of magnetometers 41, 42 and 43 aretransmitted by a transceiver to an examination station 7 which containsa special fall-detection processor 8 whose operation will be describedbelow. Initially, when a fall is detected, it is transmitted to a localdisplay device 9 to raise the alarm and to an alarm device 10 which mayinclude lights 11, sirens etc. and a button 12. The alarm device 10 islocated in a place visible by person 1 who has the option of cancellinga false alarm by pressing button 12. In its basic form, the alarm islinked to a remote medical assistance unit which can activate the rescueof person 1. In a more advanced and useful version, processing isengaged: the alarm-control device is situated on the patient's body.

A strong variation in amplitude, either of duration or frequency,measured by the accelerometers and especially the verticalaccelerometer, is associated with a movement inflicting an impact on thesensor (fall, step, jump, etc.). A change in the value of themagnetic-field projection along the sensor's axes is associated with achange in the person's orientation, such as in a fall, going to bed, orthe action of bending down.

The occurrence of the two events in a short space of time, of the orderof a few seconds, for example 2 seconds, is a strong indication of afall.

The sensor measurements may be those given in FIG. 3 in which the firstdiagram 3A shows the readings of accelerometer 31 and of magnetometer 41in the vertical axis VT, diagram 3B shows the readings of accelerometer32 and of magnetometer 42 in the medio-lateral axis ML, and diagram 3Cshows the readings of accelerometer 33 and of magnetometer 43 in theantero-posterior axis AP. The sensor's range of measurements is about ±5g for the accelerometers and about 50 micro-Teslas for themagnetometers. There are detectors on the market which can be usedwithout any particular difficulty. An example is the tri-axial LIS3L02AQaccelerometer from ST Micro Electronics. In this example theaccelerometer readings yield mainly periods of stability punctuated byshorter periods of instability in which an acceleration causes short butsignificant oscillations. These periods of instability number two hereand correspond to two different events: the first, recorded at abouttime t=1000, is a leap taken by person 1; the other, recorded betweenabout t=4000 and 5000 is a fall. These two events are rather indistinctin terms of acceleration readings, especially in the vertical axis VTwhich is precisely the one which most easily identifies a fall; but itis noted that the readings from magnetometers 41, 42 and 43 almostignore the jump, whereas they show a significant change in the case ofthe fall which is particularly visible in the vertical component ofdiagram 3A. More precisely, the reading from magnetometer 41 in thevertical axis VT shows two plateaux at different levels before and afterthe fall, and this is linked to a gradual and fairly regular change inthe instability readings of the acceleration signal. Diagrams 3B and 3Creveal that a similar conclusion about the occurrence of a fall cangenerally be reached in the same way by using magnetometers 42 and 43placed along axes other than the vertical VT; however, there appears tobe an exception when the fall does not involve a change in the anglemade by the vertical axis with the direction of the ambient magneticfield, and this may mean measuring the magnetic field with at least twomagnetometers oriented in different directions for greater reliability;it may be noted here that the change in the strength of the measurementmade by magnetometer 42 in the medio-lateral axis ML is not verysignificant, and this may mean that the fall goes unnoticed if only themeasurements associated with this axis are taken into account.

Efforts are being made to obtain a good discrimination between anaccidental fall and other events resembling it in the signalstransmitted. An additional criterion which it is interesting to deployin this respect is to verify that person 1 is lying flat after thefall—and this is usually the case with only a few exceptions.

A horizontal position can be verified if accelerometer 31 in thevertical axis VT gives an effectively zero reading after the eventassumed to be a fall, which means that it is perpendicular to thedirection of gravity. This verification is made here after about timet=5000.

An effort can be made to differentiate the downward motion of person 1who lies down, particularly when going to bed. Magnetometricmeasurements are useful here too, as the azimuth of person 1 or hishorizontal orientation with respect to the ground will be uniform towithin a few degrees when he is normally in bed. A reading of theresults from magnetometers 41 and 42 arranged along the vertical axis VTand the medio-lateral axis MT is taken at the same time, approx. t=5000,to deduce the azimuth of person 1. If it differs from that of a personlying normally, a fall may be presumed.

If the reading from the final azimuth is lacking, a criterion that isalmost as interesting may be obtained by comparing the changes inazimuth between the initial and final positions.

It is also possible to use sensor 2 to recognize the normal occurrenceof going to bed. Diagrams 4A, 4B and 4C of FIG. 4 illustrate,respectively, the results from magnetometers 41, 42 and 43 along thevertical axes VT, the medio-lateral axis ML and the antero-posterioraxis AP for a series of instances when person 1 went to bed. It is notedthat the signals recorded at different times for the same action arevery similar and may yield an electronic signal for the action, asperson 1 always executes more or less the same motions. It is possibleto detect a stage when he sits down between time t=0 and approx. t=700(in diagram 4A), whilst turning (in diagram 4B) and leaning forward (atapprox. t=500 in diagram C), before turning again until t=1700 (indiagrams 4A and 4C). The signature recorded by preliminary calibrationsmay involve both parts of the event, as distinguished above, or only thesecond which is more characteristic. These signals can then be comparedwith the interesting parts of equivalent signals obtained similarly withthe same detectors and in the same environment, by using classicaltechniques for correlating signals.

FIG. 5 illustrates how the procedure works. After initialisation atstage 50, a test 51 is carried out periodically to check whether sensor2 is worn regularly. If so, the readings from sensor 2 pass to stage 52.The results are stored in the circulating memory of a processor 8 forabout five minutes. If no event is recorded in the following stage 53,the system returns to stage 51 and runs in a loop. But when an event isrecorded at stage 53, such as a sharp change in the accelerometer'ssignal, a significant change in the signals from the magnetometers orthe detection of a horizontal posture, an analysis 54 is made by thecirculating memory according to the criteria previously indicated orsome of them. In the present case these criteria may be reduced to fourdigital criteria C1, C2, C3 and C4 calculated at stage 55 and taking thevalue 0 or 1 according to whether they are deemed absent or present;intermediate values, corresponding to a fuzzy-logic analysis, may besuggested in doubtful cases.

The number of criteria used depends on the reliability of the desiredresult and on processing quality and power. The more criteria there are,the greater the reliability.

Criterion C1 detects whether the person is lying down by examining thevertical-acceleration signal. It is set at 1 if it is verified for adefined period of time. If a horizontal posture has already beenverified, this criterion must be validated irrespective of how long thesituation has obtained.

Criterion 2 corresponds to a period of vigorous activity undertaken byperson 1 changing to a state of weak activity, and this is translatedinto oscillations or significant variations in the signals. If the weakactivity is maintained for a specific period, this criterion is set at1. The acceleration signal in the vertical axis VT is used. Othersignals can be used.

Criterion 3 corresponds to a comparison of the person's azimuth afterthe event with the azimuth of the normal reclining position. If theseazimuths differ beyond a certain degree of tolerance, the criterion isset at 1. It can be seen that magnetometers 41 and 42 were used.

Lastly, criterion C4 corresponds to a comparison of the movementinherent in the event with the signatures already recorded in certaintypical and normal movements, according to the explanations given inFIG. 4. If these movements differ beyond a certain threshold, thecriterion is set at 1.

The following stage, 56, is a weighted sum of criteria C1 to C4,according to the formula:S=w1C1+w2C2+w2C2+w3C3=w3C4+w3C4,where the sum of the weighting factors w1 to w4 is equal to 1. If thesum S is greater than a certain threshold—this can be 0.5 but may beselected according to the sensitivity required and the degree ofsupervision of person 1—the alarm is triggered after the fall-detectionstage 57. The system returns to stage 51 and continues to take readingswhatever the diagnosis.

1. A process for detecting a fall of a person, comprising the followingsteps: placing a sensor on the person, said sensor comprising at leastone accelerometer and at least one magnetometer; monitoring anacceleration signal and an ambient magnetic field signal provided by thesensor; analyzing changes in said acceleration signal and said ambientmagnetic field signal; indicating a fall when a plurality of criteriahave been met, the criteria including: a) identifying the presence of afirst period of instability in a vertical component of the accelerationsignal, followed by a second period of instability in said verticalcomponent of the acceleration signal; said vertical component beingconsidered in a height direction of the person, and b) discriminatingbetween a fall and other events by analyzing a portion of the ambientmagnetic field signal during said first period and said second period.2. The process according to claim 1, wherein the portion of the ambientmagnetic field signal includes a component of the ambient magnetic fieldalong said height direction.
 3. The process according to claim 2,wherein said discriminating includes indicating that a fall did nothappen when said component of the ambient magnetic field along saidheight direction does not vary above a threshold in said portion of theambient magnetic field signal.
 4. The process according to claim 1,wherein the criteria further include: c) detecting a recumbent positionof the person if the vertical component of the acceleration signal isnear zero during the second period.
 5. The process according to claim 1,wherein said discriminating is performed by comparing said portion ofthe ambient magnetic field signal with equivalent portions of signalsrecorded previously and respectively corresponding to normal events. 6.The process according to claim 5, wherein said comparing is performedfor a part of the portion of the ambient magnetic field signal, saidpart corresponding to either the first period of instability or thesecond period of stability.
 7. The process according to claim 6, whereinsaid analyzing of the ambient magnetic field signal includes determiningan azimuth of the person.
 8. The process according to claim 7, whereinone of the normal events includes laying on a bed and said comparingincludes comparing the azimuth of the person during the second periodand an azimuth of the bed.
 9. The process according to claim 1, whereinsaid indicating comprises indicating a fall after a threshold is crossedby a weighted numerical combination of the criteria.
 10. The processaccording to claim 1, further comprising converting raw measurements ofthe sensor to express components of signal axes associated with theperson and consisting of a vertical axis, an antero-posterior axis and amedio-lateral axis.
 11. The process according to claim 1, wherein saidsensor includes at least three accelerometers that measure accelerationin three orthogonal axes and at least three magnetometers that measurethe ambient magnetic field in the three orthogonal axes.
 12. The processaccording to claim 11, wherein said sensor has a range of measurement ofabout +/−5 g for the accelerometers and about 50 micro-Telsa for themagnetometers.
 13. The process according to claim 1, wherein saidmonitoring is performed continuously.
 14. The process according to claim1, wherein said discriminating is performed by comparing said ambientmagnetic field signal before and after at least one of said first andsecond periods of instability.
 15. The process according to claim 14,wherein said discriminating is performed by identifying said fall whensaid ambient magnetic field signal shows two plateaus at differentlevels before and after said at least one of said first and secondperiods of instability.
 16. The process according to claim 14, whereinsaid discriminating is performed by identifying said fall when saidvertical component of said acceleration signal is effectively zero aftersaid at least one of said first and second periods of instability. 17.The process according to claim 1, wherein said first and second periodsof instability are defined by oscillations in said acceleration signaland are separated by a period of stability during which saidacceleration signal is relatively constant.
 18. The process according toclaim 1, wherein said step of indicating comprises transmitting a signalto a display device, wherein said signal signals that said falloccurred, and said display device produces an alarm indicating that saidfall occurred.